Our major interests are focusing on a better understanding of the processes of determination, differentiation and cell interactions occuring during embryogenesis and metamorphosis in Drosophila. Our work on embryos support the hypothesis that during earliest stages of development interactions of egg components are important to establish determination of different cell types. The imaginal discs are distinct cell groups in the larval body which are determined to differentiate particular adult structures. It has been shown that different regions within a particular disc are specified to form different regions of an appendage. Working with imaginal discs we made the following findings. 1. Within an imaginal disc the different cells are developmentally at different stages, some are more advanced than others. 2. If the cells which form the distal part of an appendage are removed the remaining proximal cells can regenerate the missing structures. To express this regenerative capacity not all proximal cells are necessary and involved. 3.Cell clones induced in growing fragments show preferential growth toward the wound area. 4. Mixing cells of different fragments reveals that they always interact but that this interaction, depending on the origin of the fragment, does not always lead to growth.